There has long been a need for a simple unitary relief valve mechanism that efficiently relieves excessive pressure or vacuum in a transportation tank or the like. There are many stringent requirements for such a valve. It must contain a number of fail-safe features. In the event of tank roll-over at any angle, for example, the valve must seal completely to prevent any leakage of the product. Additionally, such mechanism must prevent product surge from opening the valve in case of sudden stops. Internal tank vacuum must be sufficiently relieved on fast unloading. On either excessive pressure or vacuum, the action of the valve must be quick and positive. The valve must perform efficiently even with the presence of possible foreign matter in the product. Also, the valve must have maximum sealing seat life and must be unusually easy to assemble and disassemble.